In regard to a process for producing acetic acid, an industrially used production process comprises allowing methanol to continuously react with carbon monoxide in the presence of a catalyst containing a group 8 metal of the Periodic Table (such as a rhodium catalyst or an iridium catalyst), an ionic iodide (e.g., lithium iodide), and methyl iodide and in the presence of water to give acetic acid. In this process, usually, a reaction mixture obtained by carbonylation of methanol is subjected to a flash distillation, the resulting volatile component from the flash distillation is distilled in a first distillation column to form an overhead from a top of the column and a heavy component from a bottom thereof, and an acetic acid stream is withdrawn as a side stream (side cut stream) from the first distillation column. Moreover, the overhead from the first distillation column is cooled and condensed to form an aqueous phase and an organic phase, which are separated from each other; the aqueous phase mainly contains water and acetaldehyde, and the organic phase mainly contains methyl iodide. Further, the acetic acid stream is subjected to a second distillation column to remove water and other impurities for obtaining or separating a further purified acetic acid stream as a side stream (side cut stream) or bottom stream. The second distillation column is mainly used for dehydration in many cases. Since an overhead from the top of the second distillation column has a low water content, the overhead is rarely separated into two phases (an aqueous phase and an organic phase) even after cooling and condensation. In such a process, accumulation of hydrogen iodide in the first and second distillation columns deteriorates the quality of product acetic acid due to contamination with hydrogen iodide and causes corrosion of an apparatus (such as the first and second distillation columns).
In order to remove hydrogen iodide, it has been reported that hydrogen iodide is converted into methyl iodide, having a lower boiling point, by a reaction of hydrogen iodide with methanol, and the resulting methyl iodide is separated as a lower boiling point stream.
Japanese Patent Application Laid-Open Publication No. 6-40999 (JP-6-40999A, Patent Document 1) discloses that introduction of a small quantity of methanol below a feed point, at which a feeding composition is fed to a distillation zone, converts hydrogen iodide into methyl iodide which is removed as a light end stream of a distillation column.
Japanese Patent Application Laid-Open Publication No. 52-23016 (JP-52-23016A, Patent Document 2) discloses a process for removing and collecting iodine-containing components and drying acetic acid, which comprises: introducing an acetic acid stream containing water, methyl iodide and hydrogen iodide into a first distillation zone intermediate; removing methyl iodide and others as an overhead fraction from the first distillation zone; removing hydrogen iodide and others from the bottom of the first distillation zone; withdrawing a side stream (acetic acid stream) from the middle section of the first distillation zone for introducing the stream into the upper section of a second distillation zone; introducing methanol into the lower section of the second distillation zone; removing an overhead stream containing methyl iodide and others from the second distillation zone; and withdrawing a stream of a product acetic acid substantially free of hydrogen iodide and methyl iodide from the bottom or a site near to the bottom of the second distillation zone.
Japanese Patent No. 4489487 (JP-4489487B, Patent Document 3) discloses a process for separating hydrogen iodide, which comprises distilling a mixture containing hydrogen iodide, water, and a component having a boiling point higher than that of water (e.g., acetic acid) to separate hydrogen iodide, wherein an alcohol (e.g., methanol) is fed to a distillation column so that a zone having a water concentration of 5% by weight in the distillation column may be formed between feed positions of the alcohol.
In the production of acetic acid, removal of hydrogen iodide by using the relationship between a water concentration and a hydrogen iodide concentration in a distillation column is also known. For example, Great Britain Patent No. 1350726 (Patent Document 4) discloses that because of a peak concentration of hydrogen halides occurring in a middle portion of a distillation column, if a side stream is withdrawn from the middle portion of the distillation column then the hydrogen halides will be removed therefrom, in a case where a liquid composition of carboxylic acid has a water concentration ranging from 3 to 8% by weight. Further, this document discloses that a reaction product of methanol and carbon monoxide is subjected to a flash distillation and then a fraction separated by the flash distillation is introduced into the distillation column to concentrate hydrogen iodide in a side stream from the middle portion of the distillation column, thereby removing the hydrogen iodide.
Japanese Patent Application Laid-Open Publication No. 2006-160645 (JP-2006-160645A, Patent Document 5) discloses a process for producing acetic acid, which comprises: distilling a mixture containing hydrogen iodide, water, methanol, methyl iodide, acetic acid and methyl acetate in a water content of not more than 5% by weight in a distillation column, withdrawing a fraction containing hydrogen iodide from the top of the column, and withdrawing acetic acid as a side-cut stream by side-cut or a stream from the bottom of the column to reduce the concentration of hydrogen iodide to not more than 50 ppm. According to this process, distillation at a water concentration of not more than 5% by weight in the distillation system allows inhibition of concentration of hydrogen iodide in the distillation system.
This document discloses that the mixture may be distilled by introducing at least one component selected from the group consisting of methanol, methyl acetate and an alkali metal hydroxide at an appropriate position of the distillation column (for example, at the bottom, or between the bottom and the middle section) for maintaining or keeping the water content of not more than 5% by weight in the distillation column and that such a process can remove hydrogen iodide. Further, Patent document 5 discloses in Examples and Comparative Examples that a liquid mixture containing 34% by weight of methyl iodide, 9.8% by weight of methyl acetate, 1.2% by weight of water, 55% by weight of acetic acid, and 190 ppm by weight of hydrogen iodide was distilled, and the resulting distillate from the top of the column was separated into an upper layer and a lower layer.
Although these processes can inhibit the concentration of hydrogen iodide in the distillation column, the hydrogen iodide removal efficiency is still insufficient to produce a high-quality acetic acid. Moreover, even if the overhead from the distillation column is condensed, the overhead is not efficiently separated sometimes into an aqueous phase (upper phase or light phase) and an organic phase (lower phase or heavy phase). In particular, when the liquid mixture is distilled and the overhead (fraction) from the column top is cooled and condensed according to Patent Document 5, the condensate has an inefficient separability into upper and lower phases. Further, even if the condensate is separated into the lower phase and the upper phase, it is impossible to stably perform or operate these processes due to an unsteady phase boundary (liquid interface) between these phases. Thus the continuous operation of the production apparatus sometimes confronts obstruction or trouble.
Japanese Patent Application Laid-Open Publication No. 2009-501129 (JP-2009-501129A, Patent Document 6) discloses a process for producing acetic acid, which comprises: separating a reaction mixture obtained by carbonylation of methanol into a catalyst stream and an acetic acid stream in a catalyst-separating column; in a first distillation column, separating the acetic acid stream into a first overhead containing methyl iodide, methyl acetate and a portion of water, and a first higher boiling point stream containing portions of water and propionic acid, and withdrawing a first side stream containing the acetic acid by side cut; feeding the first side stream to a second distillation column; and withdrawing and collecting a second side stream containing the acetic acid by side cut. This document discloses a process for reducing a concentration of a hydrogen halide contained in a product acetic acid, which comprises converting hydrogen iodide in the distillation column into methyl iodide and separating hydrogen iodide in the form of methyl iodide from the top of the distillation column to inhibit condensation of the hydrogen halide. The method for converting hydrogen iodide includes a method for feeding the first distillation column with water or water and a first component (A) (wherein the first component (A) is at least one member selected from the group consisting of methanol and methyl acetate) and a method for feeding the first distillation column with the first component (A) from a lower position relative to a first side stream port for side-cut of a first side stream. Moreover, the document states that because supply of water to the first distillation column develops (forms) a zone having a high water concentration in the distillation column and causes condensation of hydrogen halide in the zone, supply of water together with the first component (A) allows efficient conversion of hydrogen halide into a low-boiling component.
The patent document 6 discloses, in Examples, that methanol (4.9 mol/h), methyl acetate (7.4 mol/h) and water (21.1 mol/h) were fed from the 27th plate from the top of the distillation column (having 30 plates) (Examples 3 and 4), and methanol was fed from the 43th from the top of the distillation column (having 50 plates) (Comparative Example 2).
According to the process, unfortunately, a feeding liquid has a high concentration of methyl acetate, and in addition, the concentration of methyl acetate in an overhead fraction or stream (vapor phase component) from the top of the distillation column is further increased due to further condensation of methyl acetate in the distillation column and by-product methyl acetate formed by feeding of methanol. Thus, if the fraction from the top of the distillation column is cooled, the resulting condensate cannot be separated into an aqueous phase (an upper phase mainly containing water and acetaldehyde) and an organic phase (a lower phase mainly containing methyl iodide). Moreover, even if the condensate is separated into the phases, these phases are mixed to form a mixed phase due to a small difference in specific gravity between the light phase and the heavy phase, so that the distillation column cannot be operated stably. In particular, in an industrial process, the boundary (interface) between the aqueous phase and the organic phase varies depending on the rapid expansion of carbon monoxide in the reaction system, the flow rate and pressure fluctuations in the flash distillation step and others. Therefore, the aqueous phase and the organic phase cannot be separated clearly, so that the process apparatus cannot be operated continuously. Further, feeding of methanol or methyl acetate into the column from a position lower than a plate for feeding the mixture meaninglessly increases the diameter of the distillation column, resulting in low economic efficiency.